This invention relates to the field of machine vision, and more specifically to a method and apparatus of obtaining three-dimensional inspection data for electronic parts in a manufacturing environment. The invention relates to circuitry used to control the illumination of the lamps and preventing bias and gain drift due to variances in temperature.
There is a widespread need for inspection data for electronic parts in a manufacturing environment. One common inspection method uses a video camera to acquire two-dimensional images of a device-under-test.
Height distribution of a surface can be obtained by projecting a light stripe pattern onto the surface and then reimaging the light pattern that appears on the surface. One technique for extracting this information based on taking multiple images (3 or more) of the light pattern that appears on the surface while shifting the position (phase) of the projected light stripe pattern is referred to as phase shifting interferometry, as disclosed in U.S. Pat. Nos. 4,641,972 and 4,212,073 (incorporated herein by reference).
The multiple images are usually taken using a CCD (charge-coupled device) video camera with the images being digitized and transferred to a computer where phase-shift analysis, based on images being used as xe2x80x9cbuckets,xe2x80x9d converts the information to a contour map (i.e., a three-dimensional representation) of the surface.
The techniques used to obtain the multiple images are based on methods that keep the camera and viewed surface stationary with respect to each other while moving the projected pattern.
One technique for capturing just one bucket image using a line scan camera is described in U.S. Pat. No. 4,965,665 (incorporated herein by reference).
U.S. Pat. Nos. 5,398,113 and 5,355,221 (incorporated herein by reference) disclose white-light interferometry systems which profile surfaces of objects.
In U.S. Pat. No. 5,636,025 (incorporated herein by reference), an optical measuring system is disclosed which includes a light source, gratings, lenses, and camera. A mechanical translation device moves one of the gratings in a plane parallel to a reference surface to effect a phase shift of a projected image of the grating on the contoured surface to be measured. A second mechanical translation device moves one of the lenses to effect a change in the contour interval. A first phase of the points on the contoured surface is taken, via a four-bucket algorithm, at a first contour interval. A second phase of the points is taken at a second contour interval. A control system, including a computer, determines a coarse measurement using the difference between the first and second phases. The control system further determines a fine measurement using either the first or second phase. The displacement or distance, relative to the reference plane, of each point is determined, via the control system, using the fine and coarse measurements.
Current vision inspection systems have many problems. Among the problems are that the intensity of the light received at a light receiver may vary. When the intensity of the light varies at the receiver, it is very difficult to correlate signals that are generated by the receiver. Another problem is that the light receiver may vary in temperature. Many of the receivers use a charge coupled device (also called a CCD). A charged coupled device or CCD is a high speed, high density computer storage medium in which the transfer of stored charges provides the information. The stored charges produce signals which provide image information received. CCDs tend to heat up as they operate. As the CCDs heat, the signals produced shift or vary. Typically, there are currents associated with dark areas and currents associated with light areas. Part of correlating the data obtained includes a subtraction of the current associated with a dark area. This is used to find the absolute value of the current associated with a light area. Dark currents approximately double with every 7xc2x0 C. rise in temperature. When the currents associated with dark areas get too large, the current shows up as noise. The elimination of noise is another problem which complicates processing of the image. Within a CCD there are many individual sources of light. These light sources, typically LEDs, may also vary on a pixel-by-pixel or source-by-source basis since the gain and bias may differ as a result of manufacturing variances. This too adds to the difficulty in correlating data obtained from one sensor or another sensor since light of different intensities is being produced by the source.
Another problem is that infrared light may also adversely effect the sensor. Infrared light can vary the signal strength read by the sensor. Yet another problem associated with machine vision systems is that an image is captured using a substantially constant source of light. In order to get an adequate image, the image must be captured over a relatively long time frame. The result is the same as occurs when there is low levels of light and the object to photograph is moving fast. The image is blurred or smeared.
To overcome the problems stated above as well as other problems, there is a need for an improved machine-vision system and more specifically there is a need for a circuit or circuits which can be used to control lamp brightness or the aperture associated with a CCD. There is also a need for temperature control of the sensor to prevent or lessen the effects of thermal drift on the bias or gain of the signal output from the sensor. In addition, there is a need for a system which allows for correction of a CCD or other light sources on a pixel-by-pixel basis so that manufacturing tolerances are accounted for. There is also need for a device that controls the amount of infrared light that is received at a sensor so as control the bias or gain of the signal output from the sensor. In addition, there is a need for a device which facilitates automated high-speed three-dimensional inspection of objects.
A machine-vision system for inspecting a device includes a light source for propagating light to the device and an image detector that receives light from the device. Also included is a light sensor assembly for receiving a portion of the light from the light source. The light sensor assembly produces an output signal responsive to the intensity of the light received at the light sensor assembly. A controller controls the amount of light received by the image detector to a desired intensity range in response to the output from the light sensor. The light sensor assembly includes a beam splitter positioned between the light source and the device, and a light sensor positioned to receive light from the beam splitter. The beam splitter filters infrared light from the light source. The controller controls the light intensity by controlling the power supply for the light source or by controlling the amount of light received by the image detector as the image detector acquires an image. The image detector can be an array of imaging pixels. The controller controls the amount of light received by the image detector by controlling the amount time the array of imaging pixels receives light to acquire an image. The imaging system may also include a memory device which stores correction values for at least one of the pixels in said array of imaging pixels. The correction value can be controlled by the controller.
The machine-vision system for inspecting a device may also include a cooling element attached to the imaging device. The cooling element removes heat produced by the image detector to keep the image detector within a selected temperature range. The cooling element may be a thermoelectric semiconductor. The thermoelectric semiconductor unit may include a temperature sensor for sensing the temperature of the array of semiconductor imaging pixels. The cool portion of the thermoelectric semiconductor is attached to the array of semiconductor imaging pixels and forms a thermally conductive path between the array of semiconductor imaging pixels and the thermoelectric semiconductor unit. A controller controls the amount of power input to the thermoelectric semiconductor to keep the image detector within a selected temperature range.
The machine-vision system for inspecting a device may also include a strobed light source for propagating light to the device. The image detector that receives light from the device remains in a fixed position with respect to the strobed light source. A translation element moves the strobed light source and image detector with respect to the device. The machine-vision system may include a ring light source. The machine-vision system may also include a strobed light controller which controls the strobed light source to produce light having a first level and to produce light having a second level. The strobed light controller controls the strobed light source to produce light having a first level alternated with light having a second level. The strobed light controller may control the strobed light source and the strobed ring light source. The image detector is comprised of a first line of pixels and a second line of pixels.
A method for acquiring physical information associated with of a device using a machine-vision station having a light source and having an image detector includes the steps of projecting light from the light source to the device, receiving light reflected from the device into the image detector, and controlling the amount of light received at the image detector to a value within a desired range. The step of controlling the amount of light received at the image detector includes the steps of sensing the light projected from the light source, and controlling the amount of power input to the light source in response to the value produced by said sensing step. Another way of controlling the amount of light received at the image detector includes the steps of sensing the light projected from the light source, and controlling the length of time the image detector is exposed to reflected light in response to the value produced by said sensing step. Another way to control the amount of light received at the image detector further comprises the steps of sensing the reflected light from the device, and controlling the amount of power input to the light source in response to the value produced by the sensing step. A variable aperture may also be used to control the intensity of light received at the sensor. Heat can also be removed from the image detector to keep the image detector within a selected temperature range. This prevents thermal drift of the signals output from the image detector.
Another method for acquiring physical information associated with of a device uses a machine-vision station having a light source and having an image detector. The method includes the steps of fixing the relationship between the light source and the image detector, moving the light source and the image detector with respect to the device, projecting strobed light from the light source to the device, and receiving light reflected from the device into an image detector. The step of projecting strobed light from the light source to the device further comprises the steps of producing a first level of strobed light from the light source, and producing a second level of strobed light from the light source. The strobed light may be produced by a ring light. The strobed light may be alternated between a first and second level. The inspection systems described above may be used in any manufacturing system or environment. Once such manufacturing system or environment is a semiconductor part fabrication unit which fabricates part or a whole portion of a semiconductor device.
Advantageously, this invention solves or lessens many of the problems associated with current vision inspection systems. A feedback loop is used to control the intensity of the light received at a light receiver. This reduces the variability of the light intensity received at the light receiver. This eases the task of correlating signals generated by the receiver. Within a CCD the individual sources of light may be controlled to control the gain and bias resulting from manufacturing variances. This also eases the task of correlating data obtained from one sensor or another sensor since light of different intensities is not being produced by the source. A thermoelectric device is added to the light receiver to moderate temperature changes. This reduces signal shift or variation. An infrared filter in the path before the sensor also aids in keeping the signal from the sensor correlatable since the effects of IR light are reduced. In addition, the source of light is strobed to eliminate blurred or smeared images.